Distribution and conservation status of the Corangamite Water Skink (Eulamprus tympanum marnieae) on the Victorian Volcanic Plain
Garry Peterson, Department of Zoology, La Trobe University
Environment Australia
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About the report
The Corangamite Water Skink, Eulamprus tympanum marnieae is a recently described subspecies of the Southern Water Skink, Eulamprus tympanum tympanum. Endemic to the Victoria Volcanic Plain, E. t. marnieae is a habitat specialist inhabiting localities that combine deeply fissured basaltic rock piles with remnant arboreal vegetation and permanent or ephemeral lakes east and northwest of Lake Corangamite.
Only ten extant populations of E. t. marnieae are known with most being extremely small and patchily distributed, and, with the apparent disappearance of 2 populations, the lizard is considered 'endangered'. Probable reasons for the decline and threats to existing populations include a combination of factors, including habitat loss and degradation, stochastic genetic processes and intergradation with the nominal species E. t. tympanum.
Prior to 1996, E. 1. marnieae was only known from 3 small isolated populations, two of which were assumed to be extinct. Targeted surveys to determine the distribution and abundance of E. t. marnieae, the extent of intergradation with E. t. tympanum and the proximity of these populations to E. t. tympanum in southwestern Victoria were undertaken during 1996/97 and in particular by the Corangamite Water Skink Recovery Team during 1997/98. These surveys resulted in the discovery of previously undocumented L. t. marnieae populations, as well as a number of populations intermediate in scalation and colour characters between the two subspecies. All of these newly discovered populations, as well as two of the three historical populations, were associated with regions of the Later Newer Basalts.
During 1998/99 further surveys were directed at areas of potential habitat at the western extremities and northern fringes of the Victorian Volcanic Plain. A total of 197 sites were surveyed, with the majority associated with regions of the Later Newer Basalts, or with the water bodies and watercourses that dissect the Volcanic Plain or the foothill forest and woodlands on its northern border. No additional populations of E. t. marnieae or intermediates were discovered at any of the 197 sites surveyed. E. t. tympanum, on the other hand, was found at a total of 16 sites, 8 of which were associated with 3 previously undocumented populations on the Volcanic Plain.
E. t. tympanum was associated with two regions of the Later Newer Basalts towards the western extreme of the Volcanic Plain. These two regions were within the cool temperate thermal zone of southeastern Australia and were, or historically would have been, vegetated by moist woodlands, providing an ideal environment for E. t. tympanum. The Later Newer Basalts inhabited by E. t. marnieae, on the other hand, are within the warm temperate thermal zone, which suffers dry arid summers. E. t. marnieae seems to handle these conditions by retreating into a cool moist microhabitat provided by the surrounding lakes and deeply fissured basaltic barriers.
Most other regions of Later Newer Basalts and water bodies and courses surveyed within the warm thermal zone of the Volcanic Plain lacked any substantial areas of suitable habitat, in particular large basalt rock aggregations. Suitable habitat was surveyed in a region of the Later Newer Basalts southwest of Lake Goldsmith. but no animals were found. A similar situation has also been found at some sites within a region inhabited by E. t. marnieae, and their absence from these sites and from the Lake Goldsmith sites could be due to extinction.
No intermediate populations were discovered along the northern fringes of the Volcanic Plain, and no further evidence was found to suggest that integration had previously occurred here, apart from the intermediates from Mt Emu Creek discovered during the 1997/98 survey.
The status of E. t. marnieae as an endangered taxon appears justified with its current geographical distribution restricted to three isolated regions of the Later Newer Basalts east and northwest of Lake Corangamite in central southwestern Victoria. All 10 extant populations are patchily distributed throughout these regions, with most populations small and fragmented. It seems that E. t. marnieae has recently undergone a decline with the apparent extinction of two populations, and it appears that it may also have disappeared from other sites that appear suitable both within, and just north of its current range.
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